STAMP is a transcriptional cofactor that augments the activity of the p160 coactivators SRC-1 and TIF2 in glucocorticoid receptor-regulated gene induction and repression. In earlier studies of the ternary complex of GR, the coactivator TIF2, and STAMP, we found that different regions of each protein can selectively alter none, some, or all three parameters (Amax, EC50, and PAA) for GR induction of a transiently transfected reporter gene in intact cells (Awasthi and Simons, 2012, Mol Cell Endocrinol, 355, 121-134). However, STAMP has also been described by others as a tyrosine tubulin ligase-like family member (TTLL5) with polyglutamylation activity. Furthermore, we previously reported steroid-independent activity of STAMP regarding the growth of several cell types (He et al., 2010, BMC Cancer, 10, 128). The current aim of this project is to determine the whole animal activity of STAMP. This aim has been pursued by preparing the STAMP KO mice in a homogeneous genetic background. Mice containing a disrupted Stamp gene, resulting in premature termination of translation in the middle of the protein, were prepared. Homozygous targeted mutant (Stamptm/tm) females appear normal. Stamptm/tm males also seem normal except that they are almost always sterile. This male infertility appears to result from severe defects in sperm formation that cause a high frequency of coiling of the tails and detachment of the heads along with decreased motility in the remaining intact sperm. These abnormalities are associated with disruption of sperm tail axonemes that is accompanied by the loss of one tubulin doublet in conjunction with substantial reduction in &#945;-tubulin polyglutamylation. The axonemes in other structures appear unaffected despite the fact that Stamp is the only member of the TTLL family that efficiently modifies a-tubulin by adding the first glutamyl group needed for polyglutamylation. There is no obvious change in the organs for sperm development of wt vs. Stamptm/tm males despite the levels of wt STAMP mRNA in the testes being 20-fold higher than in any other organ examined. This defect in male fertility is unrelated to any of the 24 genes previously identified as important for sperm function. Thus, STAMP appears to participate in a unique pathway of sperm maturation and motility that may be relevant for male fertility. In summary, we have used STAMP KO mice to determine the phenotypic defects of KO animals. Detailed studies of the action of the ternary complex of GR/TIF2/STAMP have established that the three parameters of GR-regulated induction (EC50, PAA, and Amax) are not coordinately regulated. Instead, different protein surfaces of the three proteins can selectively modulate individual parameters, which indicates that pharmaceuticals may exist that can alter the activities of one, two, or all three parameters. These combined findings contribute to our long-term goal of defining the action of steroid hormones at a molecular level and of understanding their role in human physiology.